A semiconductor circuit having a protection function against a reverse voltage is disclosed in JP-A-2003-124324. The semiconductor circuit includes an inner circuit and a protection circuit. The inner circuit is energized by a power source such as an exchangeable battery. The protection circuit includes a Schottky-barrier diode, which is connected in parallel to the inner circuit so that a current toward the protection circuit flows in a direction opposite to a current supplied to the inner circuit from the power source.
Accordingly, when the power source is correctly connected to the inner circuit, the current does not flow through the Schottky-barrier diode. However, when the power source is connected to the inner circuit in such a manner that the current flows reversely, the forward current flows through the Schottky-barrier diode so that the inner circuit is protected. Thus, the Schottky-barrier diode functions as a protection circuit, and electric power consumption of the protection circuit is reduced.
The Schottky-barrier diode as a protection diode is connected in parallel to a protection object circuit such as the inner circuit in such a manner that the forward direction of the protection diode is opposite to the forward direction of the protection object circuit. Thus, the protection object circuit is protected from being applied with an reverse voltage.
However, for example, when static electricity is discharged, a voltage more than a breakdown voltage of the protection diode may be instantaneously and reversely applied to the protection diode. In this case, characteristics of the protection diode may be changed, or the protection diode may malfunction. Thus, the protection diode does not function to protect the protection object circuit. Thus, it is required for the protection diode to secure the protection function.